By John HudsonJohn Hudson is a senior reporter at Foreign Policy, where he covers diplomacy and national security issues in Washington. He has reported from several geopolitical hotspots, including Ukraine, Pakistan, Malaysia, China, and Georgia. Prior to joining FP, John covered politics and global affairs for the Atlantic magazine’s news blog, the Atlantic Wire. In 2008, he covered the August war between Russia and Georgia from Tbilisi and the breakaway region of Abkhazia. He has appeared on CNN, MSNBC, BBC, C-SPAN, Fox News radio, Al Jazeera, and other broadcast outlets. He has been with the magazine since 2013.

When it comes to weapons of mass destruction, chemical weapons have long taken a backseat to nuclear weapons in the competition for public interest and non-proliferation scrutiny. But the Syrian civil war has flipped the status quo on its head, as the mere use of such weapons threatens to drag the United States into another military intervention. So when Barack Obama says "the use of chemical weapons is a game changer," what is he talking about, and why does it make a difference in a war in which 70,000 people have already died at the hands of conventional weapons?

The Science of Chemical Weapons

For starters, a chemical weapon utilizes the toxic properties of chemicals to inflict physical pain ranging from mild discomfort to death on an individual. This can include blister agents (nitrogen mustard, sulfur mustard, and lewisite) that cause eye, skin, and lung irritation; blood agents (hydrogen cyanide and cyanogen chloride) that prevent blood from transporting oxygen throughout the body; and nerve agents (tabun, sarin, and VX) that can cause instant death by shutting down the nervous system.

The type of chemical weapon is important, since not all chemicals technically qualify as "weapons" in the eyes of the international community. In this week’s widely covered attack in Syria, for instance, initial U.S. intelligence assessments have found that chlorine — not a nerve or blister agent — was used. "That would not be the same as using a chemical weapons, as defined by international treaties," notes CNN’s Barbara Starr. The difference, of course, is a chemical weapon will supposedly warrant a U.S. military response, while chlorine will only elicit more verbal hand-wringing.

Regardless, one of the main reasons the United States obsesses over the use of chemical weapons is concern about such weapons getting into the hands of terrorist groups.

The Impact of Chemical Weapons

It only takes a small amount of chemical weapons to have a devastating impact on a highly populated area. Because you get so much bang for your buck, chemical weapons have become, by far, the most widely proliferated and used weapon of mass destruction on earth.

"The military value of chemical weapons is such that the United States and the Soviet Union stockpiled tens of thousands of tons during the Cold War," notes the non-partisan Nuclear Threat Initiative group. "Countries traditionally have acquired chemical weapons before attempting to produce biological or nuclear weapons, because they are the least technologically demanding of the three. While 188 countries have joined the Chemical Weapons Convention (CWC) and have agreed not to develop, produce, stockpile, or use chemical weapons, a handful of key countries-particularly in the Middle East-remain outside of the treaty."

Syria remains one of the countries that has refused to give up its chemical weapons stockpiles, which are considered to be significant, as FP‘s John Reed reported last year. U.S. military officials have said Syria’s chemical weapons arsenal is "100 times the magnitude we experienced in Libya," and it is thought to include hundreds of tons of mustard gas, blister agents, sarin, and VX. Strategically, the United States is adamant about those chemicals not getting in the hands of neighboring terrorist groups such as Hezbollah or al Qaeda.

The History of Chemical Weapons

Chemical weapons had their coming out party during World War I, when 124,000 metric tons were used by combatants to devastating effect. Since then, chemical weapons have been used by Italy during World War II, Japan during its invasion of China, Egypt during the North Yemen Civil War, Iraq during the Iran-Iraq war, among other occasions.

But the primary example that terrifies security analysts to this day is the release of sarin by the Japanese cult Aum Shinrikyo in the mid-90s. This proved that terrorist groups were capable of creating and using sophisticated chemical weapons to horrendous effect. "The scale of the Aum Shinrikyo chemical ambitions revealed that non-state actors are fully capable of organizing and financing chemical programs," notes the Nuclear Threat Initiative." Because many chemicals commonly used in industry are themselves very toxic, terrorist organizations may also achieve their goals through the sabotage of chemical plants and shipments."

That goes a long way in explaining why the White House is willing to threaten war — or something akin to it — if Syria’s chemical weapons arsenal becomes active.